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.Dd May 17, 2025
.Dt DUP3 3
.Os
.Sh NAME
.Nm dup3
.Nd duplicate an existing file descriptor
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In fcntl.h
.In unistd.h
.Ft int
.Fn dup3 "int oldd" "int newd" "int flags"
.Sh DESCRIPTION
The
.Fn dup3
function
duplicates an existing object descriptor
while allowing the value of the new descriptor to be specified.
.Pp
The close-on-exec flag on the new file descriptor is determined by the
.Dv O_CLOEXEC
bit in
.Fa flags .
.Pp
The close-on-fork flag on the new file descriptor is determined by the
.Dv O_CLOFORK
bit in
.Fa flags .
.Pp
If
.Fa oldd
\*(Ne
.Fa newd
and
.Fa flags
== 0,
the behavior is identical to
.Li dup2(oldd, newd) .
.Pp
If
.Fa oldd
==
.Fa newd ,
then
.Fn dup3
fails, unlike
.Xr dup2 2 .
.Sh RETURN VALUES
The value -1 is returned if an error occurs.
The external variable
.Va errno
indicates the cause of the error.
.Sh ERRORS
The
.Fn dup3
function fails if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa oldd
argument is not a valid active descriptor or the
.Fa newd
argument is negative or exceeds the maximum allowable descriptor number
.It Bq Er EINVAL
The
.Fa oldd
argument is equal to the
.Fa newd
argument.
.It Bq Er EINVAL
The
.Fa flags
argument has bits set other than
.Dv O_CLOEXEC
or
.Dv O_CLOFORK .
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr close 2 ,
.Xr dup2 2 ,
.Xr fcntl 2 ,
.Xr getdtablesize 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr socketpair 2
.Sh STANDARDS
The
.Fn dup3
function does not conform to any standard.
.Sh HISTORY
The
.Fn dup3
function appeared in
.Fx 10.0 .
The
.Dv O_CLOFORK
flag appeared in
.Fx 15.0 .
